Quantification of intracardiac shunts in congenital heart disease in man has usually been achieved in the past by the invasive techniques at catheterization. Recently radioisotopic techniques have been useful in detecting and quantitating these shunts noninvasively. Little effort has been directed toward studying the effect of simultaneous congenital defects on reproducibility and accuracy of shunt ratio quantified with radioisotopic technique. A group of dogs (N equals 16) is being examined after surgical creation of an atrial septal defect and again after creation of a pulmonary artery band or mitral valve incompetence. Each shunt ratio determination is being performed with radioisotopic, electromagnetic flow probe and oximetric techniques to determine reproducibility and accuracy of each method. Determination of the effect of a second intracardiac defect on the reproducibility and accuracy of each technique is in progress. This noninvasive technique may allow more reliable shunt ratio determination.